Most semiconductor devices in use today are based on silicon substrates. As the scale of semiconductor devices progresses toward smaller and smaller dimension, the constraints on the materials used becomes increasingly critical. As semiconductor devices are generally constructed by depositing crystalline layers on a substrate, minute imperfections in the substrate crystal may be amplified with subsequent layer deposition, and may lead to gross error in the device structure. In the case of very large scale integrated circuits (VSLI) such as microprocessor chips, such errors may render the resulting chip unusable. Thus, production of monocrystalline silicon substrate wafers is essential to the industry. The substrate wafers must be essentially free of impurities and crystal defects.
Most of the electronics grade polycrystalline silicon currently used is produced by the Siemens process. A chlorosilane-hydrogen gas mixture is passed over a hot silicon filament, where elemental silicon deposits to form an ingot of polycrystalline Si. In a variation of this process, the chlorosilane is first decomposed to silane (SiH.sub.4) before contact with the filament. In another process, SiF.sub.4 is converted into SiH.sub.4, which is then decomposed to Si in a fluidized bed reactor, or directly reduced with sodium metal. The polycrystalline product is then melted and slowly recrystallized to form a monocrystalline product.
However, filament-based processes are inherently limited in efficiency. These are batch processes, and require nonproductive heating and cooling periods while the reactor is loaded and unloaded. Even the best Siemens reactors require 125-180 kW-hr per Kg Si produced. Fluidized bed reactors are generally heated externally, which results in Si deposition on the reactor walls, leading to cracks and degradation of the reactor. Additionally, the need for a separate recrystallization step introduces a further opportunity for contamination.